1. Field of the Invention
The present invention relates generally to thread gauges, and more particularly to an improved gauge for checking thread lead error.
2. Description of the Prior Art
Lead is defined as the distance from a point on a screw thread to a corresponding point on the next thread measured parallel to the axis of the threaded member. Lead is typically measured over more than one thread so that any errors in lead will be compounded or multiplied by the number of threads and thus become more apparent. Since standard thread configurations have either an integral, for example 8 or 12 number of threads per inch or a half integral, for example 51/2 or 111/2, number of threads per inch, lead is typically measured over some integral number of inches, usually one or two inches.
There have been developed a number of gauges for measuring thread lead error. One such gauge is disclosed in U.S. Pat. No. 2,937,453. The gauge of the '453 patent includes a pair of spaced apart fixed points and a third point that is movably mounted for translation back and forth on an axis that is perpendicular to and bisects the axis between the two fixed points. A spring is provided for biasing the movable point away from the fixed points and a dial indicator is provided for measuring small movements of the movable point. The fixed points are adapted to engage one thread and the movable point is adapted to engage another thread some distance away from the thread engaged by the fixed points. The gauge is calibrated such that when the distance along the axis of translation of the movable point is precisely, for example, one inch, the dial indicator reads zero and there is no lead error. However, if the distance along the axis of translation is more than or less than one inch, the amount by which that distance differs from one inch, which is the lead error, is indicated on the dial indicator.
The gauge of the '453 patent has a number of errors inherent in its design. The errors result from the position of the two fixed points. The fixed points align with the helix angle of the thread, and hence a line perpendicular to the line connecting to the two points is not parallel to the axis of the threaded member. Thus, the movable point measures along a nonparallel line and does not measure true lead error. Additionally, when the gauge of the '453 patent is placed on the threaded member, the distance between the two fixed points defines a chord of a circle. The line of intersection between the line between the tips of the fixed points and the line of translation of the movable point is thus on the chord which is spaced apart from the surface of the threaded member. The movable point, on the other hand, is on the surface of the threaded member. Thus, the gauge of the '453 patent has a tilt with respect to the surface of the threaded member. The magnitude of the tilt error becomes more severe as the diameter of the threaded member is decreased.
Other types of lead gauges are those manufactured by Allen Gauge and Tool Company, as illustrated on Pages 8 and 9 of the Allen Gauge and Tool Company catalog, published 1977. The Allen gauges generally include a fixed point and a spaced apart movable point. The spacing between the fixed and movable points may be set at an integral or half integral number of inches. A complicated linkage and very special indicator system is provided to check lead error.
The Allen gauges have a number of shortcomings. Different gauge designs are required to measure external threads and internal threads in pipe or coupling sizes smaller than four inch nominal. Additionally, a different gauge is required for measuring buttress threads than is required for measuring threads having symmetrical flanks. Thus, several gauges are required in order to measure all sizes and styles of threads.
It is therefore an object of the present invention to provide a thread lead gauge that overcomes the shortcomings of the prior art. More specifically, it is an object of the present invention to provide one thread lead gauge that can be used for both internal and external threads of all styles. It is a further object of the present invention to provide a thread lead gauge that offers a continuously selectable range of distances over which lead error may be measured. It is a further object of the present invention to provide a thread lead gauge that is not affected by helix angle or diameter variations. It is still a further object of the present invention to provide a lead gauge that does not require a special setting standard to compensate for different angles of taper. It is yet a further object of the present invention to provide a thread lead gauge having positive means for ensuring that the contact points of the bearing pins are of the proper size and shape and that new unworn or undamaged ball contact surfaces are presented to the flank angles of the thread. It is a further object of the present invention to provide a thread lead gauge that has built in storage space for spare parts. It is yet a further object of the present invention to provide a thread lead guage that is simple, reliable, and easy to operate.